a) Field of the Invention
The present invention relates to shift control apparatus and method for a continuously-variable transmission.
b) Description of the Related Art
In a previously proposed shift control apparatus for a belt-type continuously-variable transmission (or V-belt continuously-variable transmission, hereinafter simply referred to as continuously-variable transmission), an optimum target speed ratio is calculated in accordance with driving condition of the vehicle, and then a shift control valve is adjusted to a stroke position corresponding to the target speed ratio, by operating a stepping motor or a servomotor from a speed ratio command according to the target speed ratio. By means of this operation, the shift control valve produces and outputs a shift control hydraulic pressure corresponding to the target speed ratio. Thus, the continuously-variable transmission shifts toward the target speed ratio, in response to the shift control pressure. Thereby, stepless shift of the continuously-variable transmission is achieved.
A Japanese Patent Application Provisional Publication No. H11(1999)-30327 published on Feb. 2, 1999 exemplifies a previously proposed technique in a vehicle equipped with the continuously-variable transmission. In this technique, if the shift control system of a shift actuator for the continuously-variable transmission is under abnormal condition, the speed ratio is fixed to its value at the instant of determination of the abnormal condition.